The present invention relates to a method of filtering suspension and an apparatus for realizing said method. The present invention may be applied for filtering green liquor containing finely divided matter, generated, for example, in the chemical circulation of a pulp mill.
A significant subprocess in the manufacture of sulphate cellulose is the recovery of cooking chemicals. Part of said regeneration is formed by the manufacture of white liquor by causticizing, whereby lime milk and green liquor are allowed to react in order to form lime sludge and white liquor. The green liquor is generated when a chemical melt containing sodium chemicals is led from the bottom of the furnace of a soda recovery boiler to a separate dissolving vessel, in which the melt is dissolved in a dilute liquor. The most significant sodium chemicals of the green liquor are sodium carbonate and sodium sulphide. The green liquor also contains insoluble compounds, such as metal oxides, silicates, soot and other impurities. Said dregs containing impurities must be discharged from chemical circulation, since the dregs otherwise concentrates in the chemical circulation and disturbs the manufacture of white liquor in causticizing. The cleaning of green liquor is generally performed in clarifiers. When the mills become larger and the environmental regulations more stringent cleaning by sedimentation principle results in large apparatuses, the diameters of which exceed 30 meters. Moreover, clarifiers are always liable to disturbances, whereby too impure green liquor is obtained even from the correctly dimensioned apparatuses.
An alternative method is to clean green liquor by filtering. Since the dregs mainly consist of finely divided matter, the filterability of which is poor and which rapidly clog the filtering surface, said method :results in a filtering apparatus, which has a large surface area and is thus expensive. A cake containing fine particles must thus be removed from the surface often, whereby it, of course, remains thin. When such a cake is washed off, a very dilute sludge is obtained, which must be further concentrated in several process stages.
In order to improve the filterability, for example, lime sludge obtained from the causticizing process is used as filtration aid, by means of which a filtering layer having good filtering capacity when filtering finely divided green liquor is formed on the surface of the filter element. Said method results, however, in additive costs and increased amount of waste being transported to a landfill site.
The filterability of a suspension, which is difficult to filter, has been suggested to be improved also in such a way that the formation of a filter cake on the filtering surfaces is prevented by removing separated solids by causing a strong shear force on the suspension close to the filtering surface, which mixes solids back to the suspension to be filtered. The problem with this cross-flow filtration has been large liquid volumes, which must be circulated in order to generate sufficient velocity and turbulence to remove the separated solids from the filtering area.
The purpose of the present invention is to provide a filtering method and apparatus, by means of which said disadvantages may be eliminated. The present invention provides thus a simpler and more economic method and apparatus for filtering liquid-solids suspensions, especially those which have so far been difficult to filter. Although the background of the problem has been discussed above in view of the green liquor, it is apparent that it is only an example and that the invention may be applied to be used also for filtering other liquid-solids suspensions, such as kaolin sludge, clarified white liquor, soda liquor, bleaching effluent of cellulose and white water.
In the present filtering method the suspension to be filtered is brought into contact with the filtering surface of the filter element, whereby due to the pressure difference across the filtering surface the filtrate flows through the filtering surface and the separated solids substantially remains in the suspension. It is a characteristic feature of the invention that the suspension to be filtered is brought to flow downwards on the filtering surface, whereby the filtration takes place from the film flowing on the filtering surface.
An apparatus in accordance with the present invention comprises a number of filter elements, through the filtering surface of which the filtrate flows while the separated solids substantially remain in the suspension, whereby the filter is provided with at least means for passing the suspension to be filtered to the filtering surfaces and for removing the filtrate. It is a characteristic feature of the invention that the means for passing the suspension to be filtered are arranged in such a way that the suspension is brought to the upper part of each filtering surface to flow downwards on the filtering surface.
According to a preferred embodiment a filter is provided with means for recirculating the non-filtered suspension to the upper part of the filtering surface.
The present invention realizes the cross-flow filtration in a new manner, whereby shear forces preventing generation of solids cake on the filtering surface is carried out by passing suspension to be filtered to flow due to the gravitational force during the filtration down on the filtering surface. Due to the pressure difference part of the liquid is filtered there from the falling film through the filtering surface.
A continuous downwards flowing liquid film is most suitably generated by recirculating non-filtered suspension to the upper part of the filtering surface and the suspension to be filtered is supplied and mixed to the circulation flow. The generation of an even liquid film on the filtering surface requires rather large liquid amounts, generally larger than what flows into the apparatus, and that is why the recirculation of the suspension is necessary for having an efficient filtration.
All arrangements, in which the invention may be realized, are applicable for filter element in a filter according to the present invention. Such elements are most preferably a lamella, a tube and a disc. Most usually the filtrate is discharged through a filter channel inside the element, such as a tube. If so desired, the suspension to be filtered may be passed into the element, whereby the filtration takes place in an opposite direction.
The separated solids tend to accumulate on the filtering surface. By maintaining the flow velocity high enough the generation of the solids layer may be completely prevented, since separating solid particles are entrained with the falling liquid. If a solids layer tends to be generating, it may be prevented by lowering the pressure difference for a moment, whereby the flow through the filtering surface ends or slows down and the "grip" of the layer from the filtering surface loosens and it gets loose to flow with the suspension flow. In order to remove the harmful solids cake flow in the falling film may be intensified also locally in a restricted area at a time. The increase of the flow is generated by means of one or more liquid jets.
The attachment of solids on the filtering surface may also be prevented by vibration. There are both mechanic and acoustic of such vibration apparatuses known per se.
The suspension to be filtered is brought to distribute on the filtering surface in the same way as in the falling film evaporators. A liquid distribution tray, a perforated plate or like device, to which the suspension to be filtered is passed and the bottom of which is provided with openings, through which the suspension is allowed to flow evenly on the filtering surfaces, may be arranged above the filter elements.
The pressure difference across the filtering surface may be generated by joining the filter to an apparatus known per se in order to generate a vacuum. The filter elements may thereby be assembled in an atmospheric vessel or freely in atmosphere.
A required pressure difference may be maintained also by pressurizing the pressure vessel filter with gas. The gas may be arranged in a closed circulation system. The applied gas may be inert or reactive relative to the suspension to be filtered. Reactive gas is added, when so desired, during the filtration in order to realize desired reactions.
The gas possibly penetrating the filtering surface may be separated from the filtrate either in the filter element by removing it from a separate conduit than the filtrate, most preferably from above the liquid surface through the upper part of the element, or in a separate vessel outside the filter.
The filtration may take place either continuously or batchwise. In continuous filtration new suspension is supplied, and thickened suspension is discharged continuously. If, however, as clean dregs as possible are desired to be obtained, which with green liquor means efficient alkali removal from the dregs, it is possible to perform the filtration batchwise.
In batch process non-filtered suspension is not discharged continuously, but the feed of the suspension is stopped during the filtration and the suspension is allowed to be thickened by recirculating. Thereafter washing water is supplied to the filter and filtration continues. The washing water being mixed with the dregs removes impurities from the dregs, according to the above mentioned example, alkali. At the same time the water penetrating the filtering surface washes the filtering surface improving the filtration capacity. The dregs are removed as sludge to be further treated and a new filtration cycle may be begun.
An even more efficient treatment of dregs is provided, if a cake is formed of dregs for washing. In this kind of process the apparatus is provided with a separate sludge vessel. There the recirculation pump draws the suspension to be recirculated from the sludge vessel, to which the sludge flows from the bottom of the filter. The process is started by filling the sludge vessel with suspension to be filtered, whereafter the recirculation and filtration of the suspension is started. Sludge from the filter is not removed from the sludge vessel, but it is allowed to thicken there, which takes place when liquid is removed as filtrate. When the solids content of the sludge is as desired, the bottom conduit of the filter is closed and the filter is allowed to be filled with sludge. The pressure difference still prevails in a full filter, whereby liquid is infiltrated through the filtering surface, on which a solids cake is generated. After the filtration the sludge remained between the filter elements or otherwise in the filter is removed and brought back to the sludge vessel. When the bottom of the filter is closed, a cake remains due to the pressure difference on the surface of the elements, from where the gas flowing through the cake and the filter element removes humidity. The generated cake is removed by washing with water and blowing with gas to be passed for further treatment (washing and drying).
In connection with such a batch process it is possible also to carry out dregs washing, now as an efficient displacement washing. So far it has been necessary to carry out the dregs washing in separate apparatuses. The washing takes place by filling the filter with water after the drying of the cake and by emptying the filtered water then to the washing vessel. The cake is finally removed, as above, by washing with water and blowing with gas and the dregs-water mixture is passed for further treatment.
If so desired the whole treatment of the material to be filtered, such as green liquor, is desired to be performed in the same apparatus (filter) the previous alternative may be further developed in such a way that after the washing and drying the cake is loosened dry by blowing and removed, for example, through the opened lower end of the filter.
In batch process it is possible to adjust the capacity by pressure difference or by extending the interval between the cycles.